I believe you would only be automatically charged if you had made an offer to the seller first and if at the time of your offer you were prompted for a payment method -- or if eBay indicated that your default payment method would be charged and gave you an opportunity to change it.
If the seller is just making you an offer out of the blue, you should have an opportunity to select the payment method once you accept the offer (provided that eBay has not changed things again recently, which, unfortunately, is always possible).
